Blood pressure is the pressure exerted by the blood on the vessel wall. It is a principal vital sign. Various methods, both invasive and non-invasive, have been developed for its measurement.
Invasive methods are employed by penetrating the arterial wall and are typically restricted to critically ill patients. Non-invasive methods are much more common and generally involve the use of an inflatable cuff.
The standard non-invasive method is auscultation. This method measures systolic and diastolic pressures (SP and DP) by occluding a brachial artery with an inflatable cuff and then detecting the Korotkoff sounds during the deflation period with a stethoscope while observing the pressure inside the cuff with a sphygmomanometer. However, this method requires a trained operator to make the measurement.
The most widely used automated and non-invasive method is perhaps oscillometry. This method determines SP, DP, and mean pressure (MP) using an inflatable cuff, which acts as both an external pressure applicator and a blood volume sensor. More specifically, as shown in FIG. 1a, the cuff is likewise placed over usually a brachial artery and inflated to a supra-SP level (e.g., 180 mmHg) and then slowly deflated to a sub-DP level (e.g., 50 mmHg). So, during the deflation period, the brachial artery experiences trans-mural pressures ranging from negative to positive values. Since arterial compliance changes considerably around zero trans-mural pressure, the amplitude of the blood volume oscillation (due to the heart beat) varies greatly. This variation accordingly alters the amplitude of the resulting pressure oscillation that is sensed inside the cuff as illustrated in FIG. 1b. The blood pressure values are then estimated from this oscillometric cuff pressure waveform (i.e., the waveform indicating the time evolution of the pressure inside the cuff during inflation and deflation of the cuff as shown in FIG. 1a).
The original and most popular blood pressure estimation method is as follows. First, the oscillometric cuff pressure waveform is high-pass filtered as shown in FIG. 1b. Then, the envelope of the high-pass filtered waveform is determined as also shown in FIG. 1b. Next, since the arterial compliance becomes maximal when unloaded (i.e., at zero trans-mural pressure), MP is estimated as the cuff pressure at which the envelope is maximal as shown in FIG. 1. SP and DP are then estimated as the cuff pressures at which the amplitude of the envelope is some ratio of its maximum value. The ratios are fixed to empirically selected values (e.g., 0.61 before the envelope maximum occurs for SP and 0.74 after the envelope maximum occurs for DP as shown in FIG. 1) rather than being specific to the patient at the time of measurement. As a result, this “fixed-ratio” method is heuristic and can be very inaccurate. The method may be especially error prone with arterial stiffening (i.e., decrease in arterial compliance around zero trans-mural pressure) and changes in pulse pressure (PP=SP−DP).
Numerous methods have been developed to improve upon the fixed-ratio method. These methods can be categorized into at least four groups.
One group of methods employs more than one cuff. However, these methods are obviously less practical.
A second group of methods seeks to obtain a more accurate or more complete high-pass filtered waveform envelope. However, the fixed-ratio method is then used to determine the BP values.
A third group of methods apply methods different from the fixed-ratio method to estimate the blood pressure values. One method uses the phase spectrum of the oscillometric cuff pressure waveform. This method, by itself, cannot estimate MP. In addition, the method likewise resorts to empirical means to estimate SP and DP from the phase spectrum variations and may therefore yield no improvement in accuracy. Another method analyzes the shape of each beat of the oscillometric cuff pressure waveform. In particular, the duty ratio is calculated as the ratio of the non-flat duration of the beat to the entire duration of the beat. This ratio increases as the pressure applied by the cuff decreases, since trans-mural pressure increases and more oscillatory components can be observed. SP and DP are then estimated from the duty ratio using population statistics. Hence, the method is similarly empirical and may not improve accuracy.
A fourth group of methods estimates the entire blood pressure waveform rather than just DP, MP, and SP. First, DP, MP, and SP are estimated from the oscillometric cuff pressure waveform. Then, the cuff pressure waveform is measured at a constant cuff pressure, which is usually sub-DP (e.g., 60 mmHg). Finally, this waveform is calibrated with the SP, MP, and/or DP to arrive at an estimated blood pressure waveform. However, the waveform that is measured is, in fact, a blood volume waveform. Further, the arterial compliance is nonlinear. Hence, blood volume is, in general, not linearly related to blood pressure.
A more accurate method for automated and non-invasive measurement of the blood pressure waveform is finger-cuff photoplethysmography (PPG). This less popular method employs a finger-cuff with a PPG (which measures a blood volume waveform) embedded in it and the arterial unloading principle. First, the cuff is likewise inflated and deflated while measuring the PPG to yield a finger oscillometric blood volume waveform. Then, the blood volume at which the artery is unloaded is estimated. One possible way is to find the average blood volume at which the amplitude of the oscillometric blood volume waveform envelope is maximal during the deflation period. Finally, the cuff pressure is continuously varied so as to maintain this blood volume throughout the cardiac cycle via a fast, servo-control system. In this way, the cuff pressure equals the pressure inside the artery. Since the unloaded blood volume can change (e.g., due to vasomotor tone), it must be estimated periodically. However, the need for the sophisticated servo-control system makes this method prohibitively expensive. Further, the continual unloading of the artery restricts blood flow to the finger. As a result, subjects often cannot tolerate the method for very long time periods (e.g., at most on the order of hours).
In sum, blood pressure estimation from the oscillometric cuff pressure waveform is empirical and therefore generally error prone, while blood pressure measurement via the arterial unloading principle is expensive and inconvenient. Methods and apparatus are needed to overcome these limitations and thereby improve blood pressure measurement.
This section provides background information related to the present disclosure which is not necessarily prior art.